Commonplace today are high-speed communication systems that connect computer users together. Networks allow users to share data and work cooperatively. At a physical level, these networks have cables that connect together user's stations, and these cables are in turn connected together using relays or switches. Traditional electro-mechanical relays are being replaced by solid-state relays and bus switches.
Bus switches are semiconductor integrated circuits (IC's) that use metal-oxide semiconductor (MOS) transistors to make or break the connection. Several switches may be combined on a single silicon die. One such device is made by the assignee and marketed as the P15C3861 Bus Switch. More background on bus switches can be found in "Parallel Micro-Relay Bus Switch for Computer Network Communication with Reduced Crosstalk and Low On-Resistance using Charge Pumps", assigned to Pericom Semiconductor and Hewlett-Packard Company, U.S. Pat. No. 5,808,502, U.S. Ser. No. 08/622,703.
FIG. 1 shows a prior-art bus switch device. N-channel transistor 10 conducts current from its drain to its source, connecting signal lines from two buses when an enable signal is applied to the gate of n-channel transistor 10. Bus switches are usually large in size to allow a large amount of current to flow, and to provide a low on resistance.
While such an NMOS bus switch is effective for 5-volt systems, newer 3-volt systems have lower noise margins. When the gate of n-channel transistor 10 is driven to the 3-volt power supply, a voltage drop of a threshold voltage occurs across the channel. Thus a 3-volt signal applied to the drain of transistor 10 is degraded to a 2-volt signal at its source. Other devices on the bus may require TTL input-voltages. These TTL devices require a high voltage of at least 2.0 volts, leaving no noise margin for voltage drops across the bus.
Future reductions in supply voltage will make the use of simple NMOS bus switches impossible. One solution is to use a charge pump or DC-DC converter to generate a boosted voltage above the 3-volt supply, and to apply this boosted voltage to the gate of the NMOS bus switch. See for example "Voltage Booster with Pulsed Initial Charging and Delayed Capacitive Boost Using Charge-Pumped Delay Line", assigned to Pericom Semiconductor, U.S. Ser. No. 08/990,894 filed Dec. 15, 1997, Docket No. PS-20. Such DC-DC converters draw current and may not be able to meet speed requirements.
A p-channel transistors can be connected in parallel to the n-channel transistor to form a complementary metal-oxide-semiconductor (CMOS) bus switch. FIG. 2 shows p-channel transistor 12 connected in parallel with n-channel transistor 10 to form a CMOS bus switch. An enable signal is applied to the gate of n-channel transistor 10. An inverter generates the inverse of the enable signal, which is applied to the gate of p-channel transistor 12. Thus both transistors 10, 12 are enabled or disabled at the same time.
A CMOS bus switch does not develop a voltage drop across the source and drain terminals, even when reduced power supplies are used. For high signals when n-channel transistor 10 becomes saturated, p-channel transistor 12 is still in the linear region of operation and thus passes a full 3-volt signal across its channel without the threshold-voltage drop experienced by an n-channel transistor.
FIG. 3 shows the on-resistance across NMOS and CMOS bus switches. On-resistance 16 from source to drain through the transistor's channel varies with the drain voltage for the NMOS bus switch. On-resistance 16 rises sharply as the saturation voltage is reached. In contrast, on-resistance 14 for the CMOS bus switch is relatively constant for all drain voltages, since the p-channel transistor becomes more conductive to compensate for the n-channel transistor becoming less conductive as the drain voltage is increased.
Live Insertion--FIG. 4
Modern networking equipment is often reconfigured. It is desirable to add network boards or cards to a backplane bus without powering down the bus and thus shutting down the network. This is known as hot insertion or live insertion. FIGS. 4A-4C illustrate live insertion.
In FIG. 4A, hot bus 20 is a network bus such as a backplane bus in a chassis or equipment rack. Hot bus 20 is powered up and active, having signals in high and low states. These signals may be changing rapidly during the insertion sequence.
A network card is to be inserted into a slot in the chassis, and a connector on the card is to be plugged into a connector on the chassis connected to hot bus 20. The network card includes interface circuitry 22 and bus switch 18. Since no power has yet been applied to the network card, both interface circuitry 22 and bus switch 18 are powered down, with their power supply VDD floating or grounded at 0 volts.
In FIG. 4B, the network card has been inserted into the chassis, and the connectors plugged together. Bus switch 18 is electrically connected to hot bus 20. Bus switch 18 must electrically isolate hot bus 20 from interface circuitry 22, even though power has not yet been applied to interface circuitry 22 or even to bus switch 18.
In FIG. 4C, the inserted network card is powered up. The card's internal power supply VDD reaches 3 volts in a few milliseconds after plugging the card into the connector. However, during these few milliseconds, hot bus 20 must be isolated from interface circuitry 22 by bus switch 18; otherwise the signals on hot bus 20 can be disturbed. Data on hot bus 20 can be lost since high data rates use only a few microseconds or nanoseconds for each data transfer.
Once powered up, interface circuitry 22 can connect to hot bus 20 by enabling bus switch 18. An enable signal is generated by control logic in interface circuitry 22 or other logic on the inserted network card.
NMOS bus switches are ideal for live-insertion applications, since n-channel transistors do not conduct when their gates are grounded. The drains of n-channel transistors can be directly connected to the hot bus since the p-type substrates are also grounded, preventing the forward-biasing of any p-n junctions.
PMOS Bus Switch Latches Up During Live Insertion--FIG. 5
CMOS bus switches pose several problems for live insertion since p-channel transistors conduct current when their gates are grounded. FIG. 5 shows how a p-channel transistor in a CMOS bus switch can latch up during live insertion. During live insertion, as shown in FIG. 4B, the hot bus has some high signals while the interface circuitry and the bus switch are powered down. Most or all signals in powered-down circuitry is at zero volts, even when floating.
Thus the hot-bus side of the CMOS bus switch can be high, at 3 volts, while the other side is powered-down at ground. While n-channel transistor 10 does not conduct since its gate is also at ground, p-channel transistor 12 can conduct current from hot bus 20 when its gate is at ground. Even as the bus switch is powered up, p-channel transistor 12 can continue conducting current from the hot bus until its gate reaches 2 or 3 volts.
An even more serious problem is that the drain of p-channel transistor 12 can initiate latch up. The p+ drain is connected to the hot bus, which may be high at 3 volts. The N-well under p-channel transistor 12 is grounded when powered down. The P+ drainto-N-well diffusions form a p-n diode that is forward biased. Since the N-well is rather large with many capacitances, it may be slow to power up to 3 volts. Thus latch up can occur during power up of CMOS bus switches. Even if latch up is not fully developed during power up, the forward biased p-n junction can discharge the hot bus. Additionally, when power is disconnected, these diodes pull the bus to one diode drop above ground, interfering with the normal operation of the hot bus.
The hot bus can be disturbed, causing data loss, when current is connected through p-channel transistor 12, or through the forward-biased p-n junction. Thus CMOS bus switches are difficult to use in live insertion applications.
What is desired is to use a CMOS bus switch for live insertion applications. It is desired to use a CMOS bus switch that is powered down for insertion into a hot, live bus without disturbing the hot bus. A self-isolating CMOS bus switch that isolates even without power being applied is desired.